Triarylmethane compounds are known as Leucotriarylmethane dyes which include, for example, Leucomalachite Green, Leucocrystal Violet and the like. Recently, these compounds have come to be useful as dye precursor for pressure-sensitive or heat-sensitive recording paper.
Conventionally, there have been two typical methods for the synthesis of triarylmethane compounds. By way of example, the methods for the synthesis of Leucocrystal Violet (4,4',4"-tris-dimethylaminotriphenylmethane) are represented by the following two equations. ##STR1## Other triarylmethane compounds can also be synthesized according to similar procedures.
In the prior art, the aromatic aldehyde used as the starting material in the method (A) is generally prepared by the Vilsmeyer process. Specifically, an aromatic compound is reacted with the Vilsmeyer reagent, i.e. a complex of a formylating agent for aromatic compounds (e.g., dimethylformamide, diethylformamide, etc.) and an acid catalyst (e.g., phosphorus oxychloride, phosphorus oxybromide, phosgene, thionyl chloride, etc.) to form an aromatic aldehyde. Then, the catalyst is hydrolyzed and the reaction system is neutralized by the addition of an alkaline substance. Thereafter, the aromatic aldehyde is separated by filtration, washed and then subjected to further purification, if necessary.
In order to obtain an aromatic aldehyde by forming and isolating it according to the above-described procedure, a large amount of alkali is required to neutralize the acid catalyst. Moreover, the resulting aromatic aldehyde is more or less soluble in water, which leads to an unavoidable reduction in yield.
On the other hand, the method (B) involves the problem of low purity of the hydrol derivative used as the starting material. This makes it difficult to obtain the desired triarylmethane derivative in a highly pure form.